They must have run out of CD-Keys.
Guild Wars 2 is...sold out? Of digital copies? That might be a head-scratcher, but via ArenaNet's facebook page, they explain in greater detail; it's to ensure the servers aren't clogged at release. When they get into the launch, they will re-allow orders for the game. Sad for anyone who wanted to snag their copy, but such is the way online gaming works. To those of you who got your copies - best of luck!